The instant method and apparatus pertains to methods and apparatus for inhibiting corrosion of tubing in a completed well and more particularly, to such methods and apparatus which inject corrosion inhibitor into the tubing.
When fluids are produced through production tubing in a completed well, they may contain elements which corrode the tubing. Such elements often result from the combination of various gases, e.g., carbon dioxide and hydrogen sulfide, with water.
In many wells, below a certain depth, water is in vapor rather than liquid form. As fluids are produced up the well, the water vapor reaches a point in the well at which the temperature, pressure, and volume are such that the vapor condenses. It is at this point and above that the above-mentioned gases may combine with the liquid water to produce elements that corrode the tubing.
Past methods and apparatus for inhibiting corrosion include injection of a known corrosion inhibitor for a given corrosive element into the well tubing. The injection is accomplished by installing a string of injection tubing adjacent the production tubing in the well bore with the lower end of the injection tubing being in communication with the production tubing at a selected well depth. The corrosion inhibitor is pumped into the injection tubing via a pump at the surface.
There are several problems with the above-described prior art technique. The rate of corrosion inhibitor injection is arbitrarily selected. Thus, if too little inhibitor is injected, corrosion may proceed at a faster rate than if more inhibitor were injected. Alternatively, if excess amounts of inhibitor are injected corrosion is not further retarded and inhibitor is wasted. In addition, when inhibitor concentrations reach a sufficient level over the amount necessary to produce maximum corrosion inhibition, polymerization occurs when inhibitor combines with hydrocarbons and other additives such as bactericides in the well fluids. Such polymerization creates a gummy material which can plug valves.
It is a general object of the present invention to provide an improved method and apparatus which overcome the above enumerated disadvantages in the prior art.
It is a more specific object of the present invention to provide an improved method and apparatus which samples the concentration of corrosive elements in production tubing and injects a sufficient amount of corrosive inhibitor to optimize corrosion inhibition.
The method of the instant invention includes the step of monitoring the concentration of the corrosive elment in fluid produced in the well tubing. A corrosion inhibitor is injected into the tubing and the flow rate thereof is varied responsive to the monitored concentration. The injection depth is determined by calculating the depth in the well at which water vapor condenses and injecting inhibitor into the tubing at substantially that depth.